Suppose, I helped a person in his tough times. I stood by him in his financial crisis and made an all-out possible effort to help him overcome his distress. Therefore, I was surely a benefactor to him.

A few years later, he stole my undergraduate degree certificate and started blackmailing me.

What do you call a person who harms (or tries to harm) the benefactor?

In Bengali, we use the word "কৃতঘ্ন" (pronunciation: Kritoghno) for such persons. Google translate shows that the English counterpart is "Unreasonable". This is nowhere near to the idea!

I think you want an adjective rather than a noun since the Bengali translates to an adjective. "Treacherous" sounds appropriate to me.
– BoldBenMar 11 at 13:17

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Both "treacherous" and "unreasonable" do not convey the idea! A man had told me that the word, I am looking for, does not exist in literature! I wanted to verify it.
– Bishwajit PurkaysthaMar 11 at 14:37

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Also, there may be perfect terms to replace yours in English, but just not hitting the exact meanings that the Bengali word has. I'm thinking 'traitor', 'unfaithful', 'turn-coat', 'ungrateful' but can't be sure, not knowing all the nuances of the Bengali word.
– MitchMar 11 at 15:00

Show ingratitude, turn against a benefactor. For example, The college gave me a scholarship, so I shouldn't bite the hand that feeds me and criticize its hiring policies . Used about 600 b.c. by the Greek poet Sappho, this metaphor of a dog biting its master was first recorded in English in 1711.

The very fact that the definitions of this idiom do not give a single word definition suggests that there might not be a commonly used single word (other answers may prove me wrong, though!).

This is what came to my mind as well, although in my experience it's more often used as an admoishment, "Don't bite the hand that feeds you." It also feels strange to me to use it to refer to someone who helped you only in the past.
– BarmarMar 11 at 18:37

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How does this answer "what ... you call a person who harms (or tries to harm) the benefactor"?
– DanMar 12 at 9:45

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@Mitch - the OP asks (twice!) "What do you call a person who harms (or tries to harm) the benefactor?". A 'biter of the hand that feeds them' - really?
– DanMar 12 at 23:24

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@Dan: I don't know of a single noun that captures the meaning of the-ingrate-who-foolishly-bites-the-hand-that-feeds-him. Languages don't always correspond one-to-one in terms of individual words that have close connotations. To me, this phrase is the best answer, even if it does not give the OP a single noun to fill in the blank. This is how we typically express this thing in English. Second best, but it doesn't even come close even if it fills in the blank, is the noun ingrate. (I upvoted that too.)
– DrewMar 13 at 2:05

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@Drew - This is a fair point. The problem though is that this answer does not provide a label of any kind. It's clearly idiomatic to say, for example, "I wouldn't trust that guy, he bit the hand that fed him". But you wouldn't say "Don't trust that guy he's a biter of hands that feed him". Of course the phrase is related to the OP, but it describes what a person does/did, not what s/he is. The noun/noun phrase to describe such a person is the nub of this question.
– DanMar 13 at 13:08

Most would call them a "traitor" or describe them (or the deed) using a related word like "treachery or treacherous". These describe deep betrayal of trust and good will, plain and simple. See also "Judas".

"Ungrateful" seems marginally better than "unreasonable". But to capture "stealing from and blackmailing someone who helped you" I would want a word connoting evil. "Ungrateful" is benign in contrast.
– TimboMar 11 at 17:54

The Hindi word 'kritaghna' would translate to ungrateful. Although in the context of not just being ungrateful, but actually stealing/blackmailing I would use a stronger word than kritaghna/ungrateful. I am not sure if in Bengali the word has a stronger meaning.
– hojusaramMar 12 at 4:34

Ungrateful in this context would be best applied to some of the other nouns suggested here; an 'ungrateful traitor' or 'ungrateful backstabber' would help convey that the person was specifically ungrateful, indicating that they had received some benefit from the person prior to the treason. Otherwise, a traitor or backstabber might refer to someone who is otherwise a peer in the relationship, and that they have merely violated an expectation of working together rather than treated someone poorly who specifically helped them.
– Bryan KrauseMar 13 at 21:13

The idea is that by helping certain people, you become involved in their lives, and you are then included in the trouble they later make for those around them. For example, if you lend someone money they may ask you for other favours.

It does not mean you should never help people, only that you should be careful and recognise if this begins to happen.

This tends to be used humorously, and the "punishment" it refers to is usually minor inconveniences (like asking for favors). And there's also usually a cause-and-effect relationship -- in the question, it's hard to see how helping someone through a crisis caused him to steal from them.
– BarmarMar 11 at 18:42

I think unreasonable is accurate. What you define reasonable to mean essentially defines what unreasonable means: Anything not reasonable. Hurting someone who helps you sure is insane!

In today's American English, reasonable means "operating according to reason" where reason is the subset of all human logic that most humans accept as sound.

Your "Kritoghno" is someone who "bites the hand that feeds". I doubt most people would call that behavior sane, especially psychologists: Biting the hand that feeds means that you disobey the laws of psychological conditioning, meaning that you have irregular psychology. People who have irregular psychology are "insane" and do not possess the reason that everyone else possesses. Therefore, these people are called "unreasonable".

Unreasonable has much milder connotations in English than the kind of treachery described in the question. While yes, it's 'unreasonable' to do what this person did, describing it as such would be a very large understatement, almost to the point of comedy, such as calling a car accident that left one paralyzed 'a bit bothersome' An unreasonable person won't accept low-fat milk in their latte if the coffee shop is out of whole milk, for example. What is described here is well beyond unreasonable
– MarkTOMar 13 at 20:56

What flavor of English are you asking about? In American English, such a person could be described as a 'Benedict Arnold' after the historical figure, but that's not likely to be so well known outside the US. Other English-speakers may have other such references.

In the UK reasonably well known, but would probably not come to mind in this context. ;-) Quisling is probably the person who's most treachery-personified in the UK psyche, do the extent that he's sometimes exclaimed.
– DannieMar 13 at 22:49

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@Dannie Fair point. In the US, his injured leg has a monument for service during the revolution. The rest of him is reviled as a traitor :D
– MarkTOMar 14 at 16:50

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